Bubble Nebula

The Bubble Nebula, also known as NGC 7635, is a sphere of active star formation glowing faintly in the constellation Cassiopeia. The hydrogen gas cloud from which the stars form emits red light, the characteristic color of hydrogen, by absorbing energy from them. This image was made with the 0.8-meter Telescope at McDonald Observatory, with the Prime Focus Corrector instrument. Credit: Tom Montemayor/McDonald Observatory.